In recent years, in a luminaire, more and more illumination light sources are changed from an incandescent lamp and a fluorescent lamp to energy-saving and long-life light sources such as a light-emitting diode (LED). For example, new illumination light sources such as an electro-luminescence (EL) and an organic light-emitting diode (OLED) have been developed. The light output of these illumination light sources depends on a value of a flowing electric current. Therefore, when lighting is lit, a power supply circuit that supplies a constant current is necessary. Further, in order to adjust an input power supply voltage to a rated voltage of an illumination light source such as an LED, it is also necessary to convert the voltage. As a highly-efficient light source suitable for power saving and a reduction in size, a switching power supply such as a DC-DC converter is known. An LED lighting device including a self-excited DC-DC converter is proposed (see, for example, JP-A-2004-119078).
However, in the self-excited DC-DC converter, it is likely that oscillation stops if an output current flowing through an inductor is excessively reduced. Therefore, a dimming range of an illumination light source is sometimes limited.